The importance of biological autologous materials in the healing process has been well documented. Most importantly, two biological autologous materials have been shown to be directly implicated in the formation of the structure of blood clots, which provide a haemostatic barrier whose role is to ensure hemostasis and seal the wound: (1) fibrin, which derives from the separation of plasma fibrinogen into two strands through the action of thrombin, and (2) the activated membranes of platelets.
The wound healing process is generally presented as the succession of a coagulation phase, an inflammatory process and a regeneration process.
The coagulation phase (blood clotting or clot formation) is a complex process whereby a damaged blood vessel wall is covered by a fibrin clot to stop hemorrhage and the repair of the damaged vessel is initiated by the release in large quantities of cytokines and growth factors from platelet alpha granules. The formation of blood clots (formed in physiological conditions by fibrin, platelets and red blood cells, among other blood components) is a natural phenomenon that results from tissue trauma and its role in the wound healing process, as well as in the union of bone fractures, is well-known.
The inflammation process, which follows the formation of a blood clot, is stimulated by numerous vasoactive mediators and chemotactic factors (specific signals in the form of proteins) released by white blood cells and platelets. These signals attract macrophages that “clean” the site from bacteria and foreign particles as well as red blood cells before the migration of new cells.
The tissue regeneration phase involves the chemoattraction and the mitosis of the undifferentiated cells in the scaffold (or growth matrix) formed by the blood clot. The new cells which multiply under the stimulation of platelet growth factors will replace damaged or destroyed cells injured by macrophages.
Growth factors and numerous plasma proteins, also called signaling molecules, which promote cell migration and division within blood clots, play a crucial role in the wound healing process.
Theoretically, it is possible to amplify the effects of these first phases in the wound-healing cascade by discarding the red blood cells and increasing the concentration of growth factors.
Blood clotting amplification can be defined as the formation of an “enriched clot (EC)”. ECs are obtained through the use of platelet concentrates and have been described in Platelets and Megacaryocytes 2004, vol 1 & 2, as “Structure and signals”, Ed. Gibbins and Mahaut-Smith, Humana Press, New Jersey.
Platelet-rich plasma (PRP) can be defined as an autologous concentrate of platelets in a small volume of plasma; it has been developed as an autologous biomaterial and has proven to be useful in the healing and regeneration of tissues (Marx et al., 2004, J. Oral Maxillofac. Surg., 62, 489-496). PRP not only consists in a platelet concentrate but also contains growth factors (such as platelet-derived growth factor: PDGF, vascular endothelial growth factor: VEGF, transforming growth factor: TGF and epidermal growth factor: EGF) that are actively secreted by platelets and are known to have a fundamental role in wound healing initiation process.
For example, PDGF is known to initiate connective tissue healing, including bone regeneration and repair. PDGF also increases mitogenesis (healing cells), angiogenesis (endothelial mitosis into functioning capillaries) and macrophage activation. VEGF released by the leukocytes is also known to have potent angiogenic, mitogenic and vascular permeability-enhancing activities on endothelial cells. TGF-β promotes cell mitosis and differentiation for connective tissue and bone, acts on mesenchymal stem cells, preosteoblasts and fibroblasts and inhibits osteoclast formation. EGF is known to induce epithelial development and promote angiogenesis.
Platelet concentrates are generally used in dental implantology and bone surgery, notably in the USA. Various techniques of preparation of PRP by centrifugation processes have been developed. However, due to the sensitivity of the platelet cells and the variability of the efficiency of the methods of separation of the platelets from the red blood cells, a great variability exist among the methods used for the preparation of platelet concentrates (Marx et al., 2004, above; Roukis et al., Adv. Ther., 2006, 23(2):218-37): for example, the laboratory material for in vitro diagnostic which is used for platelet preparation, leads to a poor platelet and other plasma components yield (Marx et al., 2004, above: Anitua 35%, Landsberg 30%, Clinaseal 39%, ACE surgical 33%, Curasan 29%). The automated settings from Biomet PCCS & GPS (Marx et al., 2004, above), which not only present the drawback of being a complex process with prohibitive costs for the process of a blood sample, lead to only a yield of 61% and SmatPreP from Harvest Technology 62%. In those systems, there is obviously an important loss of valuable biologic tissue from the patients, therefore there is the need for the development of a reliable process collecting the plasma cells with high yields, easy to use and cost effective.
It has been recently demonstrated that the positive effects of platelet-rich plasma on bone regeneration spans a limited range of platelet concentration and revealed that an inhibitory effect occurs in the presence of more than 106 platelets per μl, which is 3 to 4 times baseline counts (Weibrich et al., 2004, Bone, 34(4):665-71).
In addition, the obtaining of platelet concentrates still needs the use of relatively complex kits and costly dedicated machinery and the equally costly involvement of specialized technicians. This drawback makes the current known methods of preparation of PRP not adapted to a point-of-care use.
Further, the preparation of cells in view of cellular or tissue regeneration for use in transplantation, post-operative regeneration or for aesthetic purpose is faced to the long-term conservation problem of cells and tissues. Tissue or cell cryoconservation is generally used for the long-term maintaining of tissues or cells, notably platelets, but this technique has shown serious drawbacks and problems such as crystal formation, osmotic problems, aggregation, inhibition of protein synthesis ability, stress protein expression in response to thermal stress, etc. Therefore, tissue or cell cryoconservation is known to alter the cell viability and stability (Agence française de sécurité sanitaire, 2003; Arnaud et al., 1999, Cryobiology, 38, 192-199; Tablin et al., 2001, Cryobiology, 43(2), 114-23). Some of the cryoconservation side effects may be limited by the use of anti-freezing agents such as DMSO or glycerol or other cryopreservatives (U.S. Pat. No. 5,5891,617, Oh et al., Cornea, 26, 840-846) but the concentration of these agents has to be adapted to limit their toxicity and side effects.
Therefore, there is a need for new or alternative method of preparation of cells and tissues suited for use extemporaneously while preserving their integrity, notably in terms of growth factors secretion ability and viability.